In general the leading approaches to magnetic confinement of plasma include closed systems (tori), open systems (magnetic mirrors), and pinches. In closed systems, the magnetic field lines (imaginary lines indicating the direction of a magnetic field at a location in space, the density of lines representing the magnitude of the magnetic field at that location; also called “lines of force”) are confined within the system, even if they do not close upon themselves; stellarators, multipoles, and tokomak reactors are examples. Open systems such as magnetic mirror machines and cusped magnetic field machines retain plasma through charged particle reflection along field lines; the 2XII magnetic mirror device developed at Lawrence Livermore National Laboratory is one example. Pinch systems, which include theta- and z-pinches (where theta and z indicate their typical cylindrical coordinate directions), confine and heat plasma by the magnetic field generated by a large plasma current; examples include theta pinch machines at Los Alamos National Laboratory and Culham Laboratory. Many variations of open and closed systems have been constructed including Astron, an open toroidal device employing a relativistic electron beam and magnetic mirrors, reversed-field configuration (RFC) devices, and others. Presently, most of the experimental investigation is on the physics of RFCs, stellerators, and tokamaks but many other confinement strategies have been envisioned and developed (see References below, Chen, 1st Ed., Ch. 9).
The primary challenge of magnetic plasma confinement is stability (Chen 1st Ed. Ch. 9). In order for plasma to be stably confined in a magnetic field it must be confined from outward diffusion by convex magnetic field lines as viewed from the plasma interior. Edward Teller in October 1954 suggested this criterion due to possible formation of “interchange” instabilities where concave magnetic field lines and plasma rapidly exchange position (see References below, Bishop). The interchange instability, and many other plasma instabilities, have since been verified experimentally and are well known to those skilled in the art (see References below, Bateman).
Despite the many unstable configurations for plasma confinement there does exist a large family of absolutely stable configurations bounded by cusped surfaces (see References below, Berkowitz 1958; Grad 1961). A cusp is a geometric term indicating a pointed end where to two curves meet, and in the practice of plasma confinement, cusp configuration magnetic fields converge between fields of reversing polarity. At these regions of convergence, plasma charged particles may either be reflected in a process analogous to magnetic mirror charged particle reflection, or lost as they travel along field lines through the cusp. Cusp reactors describe a spectrum of device configurations. Examples includes the Versatile Toroidal Facility experimental device at Massachusetts Institute of Technology, Polywell systems such as the experimental reactor presently under investigation at Energy Matter Conversion Corporation (EMC2) (see References below, Park, et al.), “picket-fence” systems (see References below, Hershkowitz and Dawson 1976), U.S. Pat. No. 2,961,559 to Marshall, U.S. Pat. No. 3,141,826 to Friedrichs and Grad, and others. Plasma processing devices for use in the semiconductor industry also use cusp-field devices for example U.S. Pat. No. 7,692,139 B2 to Koo.
Herein, reactors and devices that generate cusped magnetic fields are generally referred to as “cusped-field devices”, “cusp reactors”, & etc.
While cusped-field configuration reactors are the only known class of confinement schemes known to be absolutely magnetohydrodynamically (MHD) stable, these reactors have demonstrated significant particle losses through the cusp, described by those with skill in the art by a “hole size” (see References below, Haines), a linear dimensional measurement that may apply to line, ring, and point cusps. In part cusp losses are due to a “null point” of zero magnetic field where collisions between charged particles impart a velocity along magnetic field lines such that plasma is lost through the cusp (see References below, Berkowitz 1959). The cusp hole size has been measured to be on the order of the ion gyroradius (see References below, Allen 1965, Pechacek 1980), too large for commercial utility. In the present invention azimuthal fluid rotation forces plasma away from the confinement field null point, thus significantly modifying the physics of particle losses through the cusp.
Cusped magnetic fields demonstrate particle reflections along magnetic field lines approaching the cusp similar in physical nature to magnetic mirror particle reflection. Early theoretical work, however, assumed plasma charged particles traversed between adiabatic regions near the cusp and non-adiabatic regions away from the cusp such that adiabatic invariance common to magnetic mirror analysis did not apply (see References below, Grad 1957). The present invention possesses an adiabatic plasma sheath (see below) but whether a non-adiabatic region is necessary for particle reflection analysis is to be determined.
While cusped magnetic field charged particle reflection is similar in nature to magnetic mirror charged particle reflection, cusp losses are unique. The magnitude of the magnetic field of prior art cusp reactors does not always increase for a particle approaching a cusp (see References below, Grad 1957, and Friedrichs et. al U.S. Pat. No. 3,141,826). This gives some distinction between cusp reactors in general and the present invention that has an increasing confinement field approaching the cusp. More importantly, however, prior art cusp reactors invariably permit plasma to occupy the null point. In this region of non-adiabatic processes, particle magnetic moments are randomized, and particles accepting momentum along magnetic field lines are lost through the cusp. Attempts at “particle trapping” have not proven experimentally successful (see References below, Tuck 1959 and Grad 1960).
In cusp reactors an adiabatic plasma sheath separates field-free plasma from the vacuum magnetic field. The plasma sheath is where mirror-type particle reflections and charged particle drifts resulting in both fluid flow and current generation occur (see References below, Haines). The present invention departs from prior art cusp systems by generating a plasma fluid rotation driven by electrodes shaped to produce an electric field everywhere perpendicular to, or nearly perpendicular to, the confining magnetic field. Sheath and field-free plasma between ring cusps is thereby driven to rotate, significantly modifying the physics of prior art cusp systems (see References below, Spalding).
Additional plasma confinement reactors include U.S. Pat. No. 3,369,140 to H. P. Furth describing the annular confinement of high temperature plasmas between coaxial solenoidal field coils. This design, and others, departs from the present invention on a number of grounds. The design to Furth, and the design of U.S. Pat. No. 3,189,523 to Patrick, fail to stably confine plasma, are unable to achieve a sufficient magnetic mirror ratio, possess a concave magnetic field curvature (a shortcoming of the rotating plasma device IXION of U.S. Pat. No. 3,005,767 to Boyer and the Homopolar generator of Anderson; see References below, Anderson 1959), or are rotating shaft devices (for instance U.S. Pat. No. 4,710,660 to McKee), or in general are not cusp reactors.
To overcome the limitations of prior art cusp-field plasma confinement, the present invention introduces electrodes to drive plasma fluid rotation about the device axis, claiming the benefits of induced plasma currents.